The present invention relates to an image forming device and, more particularly, to a digital full-color image forming device which is capable of reading and storing original image data, making color corrections on the stored image and producing a color reproduction from the stored image.
The conventional digital full-color copying machine includes the combination of a scanner and a printer, may which be substituted by a printing machine and a monitor. The original image is scanned and a color correction is performed whereby the image is processed with a matrix having the color characteristic of --H reverse to the color characteristic H of the scanner and the printer. The processed colored image is printed out by the printer. The matrix of --H is selected in such a way that the color data O (r, g, b) to be printed out may be the same as those of the original.
The color correction most frequently utilizes a primary matrix and a secondary matrix which are expressed as follows: ##EQU1##
The expression (1) is a primary color correcting matrix, where r, g and b are color data to be corrected, K.sub.01, K.sub.02, K.sub.03, K.sub.11, K.sub.12, K.sub.13, K.sub.21, K.sub.22, K.sub.23 --factors, Cr, Cg, Cb--constants, r, g, b--color data after the color correction. ##EQU2##
The expression (2) is a primary color correcting matrix, where r, g and b are color data to be corrected, K.sub.01, K.sub.02, K.sub.03, K.sub.04, K.sub.05, K.sub.06, K.sub.07, K.sub.08, K.sub.09, K.sub.11, K.sub.12, K.sub.13 . . . K.sub.29 --factors, Cr, Cg, Cb--constants, r, g, b--color data after the color correction.
In the final adjustment of the full-color digital copying machines to be shipped from the manufacturer, the color correction has been made by adjusting the above-mentioned factors and constants to the values at which a minimal color difference exists between an original and a copy. Such values have usually been determined by multiple-regression analysis or a like method.
Multiple-regression analysis is one of the methods which derive a regression equation explaining dependent variables through the means of a plurality of independent variables using the method of least squares. It can be performed as follows:
To decrease the factors and constants of a secondary color correction for EQU r=K.sub.01 r+K.sub.02 g+K.sub.03 b+K.sub.04 r.sup.2 +K.sub.05 g.sup.2 +K.sub.06 b.sup.2 +K.sub.07 rg+K.sub.08 gb+K.sub.09 br+Cr,
let us minimize the following equation as to K.sub.01, K.sub.02, K.sub.03, K.sub.04, K.sub.5, K.sub.06, K.sub.07, K.sub.08, K.sub.09 and Cr: ##EQU3## Where Y(j) represents a variable in the order of J.
The minimization of the above-mentioned equation (3) means to minimize a total of the differences between the right side and the left side of the expression (2) for R determined as to every sample color Y(j), i.e., to minimize an error.
The determination of the factors and the constants by the above-mentioned multiple-regression analysis can be made in practice as follows:
A standard pattern including a variety of colors is first generated and then printed out. A printed standard pattern is input, by the use of a scanner, into the color correction unit which calculates the factors and the constants in the equation (1) or (2) by a multiple-regression analysis. The equation (2) or a higher degree equation may be considered for application in principle, but the equation (1) is mostly used to obtain printing speed and a circuit scale of the device. The determination of the factors and constants may be done, not in real time, but the practice requires real-time processing with the equation (1) or (2).
The original image is input by the scanner into the color correction unit wherein the input image is processed with a matrix --H inverse to the color characteristic H of the scanner and the printer. Thus, the processed image is printed out. The color difference between the original and its copy can be minimized by multiple-regression analysis and the equation (1) or (2) to find the inverse matrix --H so that the color O (r, g, b), to be printed out, may coincide with the original color.
Among a variety of colors, flesh-color, sward's-green and sky-blue are very familiar to us in our daily life. Therefore, in color printing, it is very important to reproduce finely these specific colors.
The following is described in a paper "Development of a color reproduction theory in hard copying", Y. MIYAKE, Journal of Electrophotography, Vol. 29, No. 3, 1990. P284-292.
In color correcting, the flesh-color area of an original copy is first extracted and the area of a face is then extracted therefrom through a process such as removing separate points, expanding, contracting, labeling and recognizing the shape. The extracted face area is corrected with a color correcting factor which assures a fine reproduction of the flesh color and the rest is corrected by a normal color correcting factor. A correcting matrix for flesh-color includes a variety of flesh-color patterns processed by multiple regression analysis in order to minimize the difference in the color between an original and a print's sample. Therefore, it can assure a fine correction of the flesh-color. However, this matrix cannot be applicable for correcting any color other than flesh-color. Accordingly, color correction is effectively done by applying the specially prepared matrix to the flesh-color area only and by applying a standard matrix to the other normal colored areas.
Original image data is input by a scanner into a color correction unit which extracts each color area of the original image from the data input therein and separately selects a suitable color correction matrix for each extracted area, e.g., a flesh-color correcting matrix for a flesh-color area and a sky-blue correcting matrix for a sky-blue area. This makes it possible to finely correct specific colors requiring precise reproduction for the human eye by selectively using corresponding matrices especially prepared for them and to limit the correction effect to only an extracted area with no effect on the rest of the areas.
Three color correcting blocks A, B and C perform color correction, respectively, with a flesh-color correcting matrix, a sky-blue color correcting matrix and a normal color correcting matrix made from a wide range of color samples. The flesh colored area and the sky blue area are separately extracted and corrected by using the color correcting matrices A (for the flesh-color area) and B (for the sky-blue color area) respectively. In the case of an example of a color image original, image data of the upper sky-blue area is corrected by the use of the color correcting matrix B, image data of the flesh-color areas of a face and hands is corrected by the use of the color correcting matrix A and image data of the remaining normal colored areas is corrected by the use of the color correcting matrix C.
A conventional type digital, full-color copying machine comprises an original photographic image, an image sensor comprising a charge coupled device (CCD), an A-D converter, a shading correction, a color converter, a portion for UCR (Under Color Removal) and BP (Black Paint) memory and laser unit.
The CCD image sensor reads by scanning the original image and transfers the image data "r", "g" and "b" (sampling) to the A-D converter which in turn converts the analog image data into digital signals R, G, B (quantized). These digital signals, which include an error induced by variations of the CCD elements and the uneven luminosity of a lamp, are corrected in the shading correction portion and then transferred to the color converter which in turn converts the digital image signals into those having a gray level suitable for the visual properties of the human eye by logarithmic conversion and converting three primary colors (R, G, B) of light into three primary colors (Y, M, C) of the toner. The UCR and BP portions perform under color removal from the converted data and by black painting thereon and then enter the processed data into the memory. When copying is required, the digital image data is subsequently read out from the memory and transferred to the laser unit which outputs a full-color image.
The Japanese publications of unexamined applications JP,A, 60-91770 and JP,A, 4-323957 describe, respectively, a color image processing device and a color image processing method, which are the prior art devices and methods which the present invention is concerned with. These prior art references involve extracting an area of a specific color from an original specimen to include even a small amount of a specific color, e.g., flesh color and conducting special processing such as fuzzy optimal processing. This may prolong the process and/or increase its performance cost in view of the fact that most of the originals have a small area of the specific color and may not be effectively processed for an increased time of the special processing. On the contrary, the present invention proposes to determine an area of a specific color of an original image and to conduct a masking process on the original color data only when the determined area of the specific color is large or to conduct normal masking of the original image if the determined area is small, thereby making it possible to quickly correct most of the normal color originals and to highly correct any original image that is rich with the specific color to get an image of high quality. This may improve the performance cost of the color image processing device.
FIG. 1 shows the principle scheme of a conventional digital full-color copying machine. The shown case includes a combination of a scanner and a printer, which, however, may be substituted by a combination of a printing machine and a monitor. An original image is scanned by the scanner to put in a color correction (step 1) wherein the image is processed with a matrix having a color characteristic of --H reversed to a color characteristic H of the scanner and the printer (step 2). The processed color image is printed out by the printer (step 3). The matrix of --H is selected in such a way that the color data O (r, g, b) to be printed out may be the same as those of the original.
As shown in FIG. 1, an original image is input by the scanner into the color correction unit wherein the input image is processed with a matrix --H, inverse to the color characteristic H, of the scanner and the printer. Thus the processed image is printed out. A color difference between the original and its copy can be minimized by multiple-regression analysis and the equation (1) or (2) to find the inverse matrix --H for making that the color O (r, g, b) to be printed out to coincide with the original color.
FIG. 2 is a view showing the system of a conventional digital, full-color copying machine.
Original image data is input by a scanner into a color correcting matrix (step 1) which extracts every color area of the original image from the data input therein (step 2) and separately selects a suitable color correcting matrix for each extracted area (step 3), and the color image is printed out by the printer (step 4), e.g., a flesh-color correcting matrix for a flesh-color area and a sky-blue correcting matrix for a sky-blue area. This makes it possible to finely correct specific colors requiring the precise reproduction for the human eye by selectively using corresponding matrices especially prepared for them and to limit the correction effect to only an extracted area with no effect on the rest of the areas.
In FIG. 2, there are shown three color correcting blocks, A, B and C which perform color corrections, respectively, with a flesh-color correcting matrix, a sky blue-color correcting matrix and a normal color correcting matrix made from a wide range of color samples.
The flesh-color and sky-blue color areas are separately extracted and corrected by using the color correcting matrices A (for the flesh-color area) and B (for the sky-blue color area) respectively. In the case of an example of a color image original shown in FIG. 3, image data of the upper sky-blue area is corrected by the use of the color correcting matrix B, image data of the flesh-color areas of a face and hands are corrected by the use of the color correcting matrix A and image data of the rest of the normal colored areas are corrected by the use of the color correcting matrix c (see FIG. 2).
FIG. 4 is a construction block-diagram of a conventional type digital full-color copying machine, wherein numeral 1 designates a photographic (image) original, and which comprises an image sensor 2 comprising a charge coupled device (CCD), an A-D converter 3, a shading correction 4, a color converter 5, a portion for UCR (Under Color Removal) and BP (Black Paint) 6, a memory 7 and a laser unit 8.
The CCD image sensor 2 reads by scanning the original image 1 (step a) and transfers the image data "r", "g" and "b" (sampling) to the A-D converter 3 (step b) which in turn converts the analog image data into digital signals R, G, B (quantized). These digital signals, which include an error induced by variations of CCD elements and the uneven luminosity of a lamp, are corrected in the shading correction unit 4 and then transferred to the color converter 5 (step c) which in turn converts the digital image signals into those having a gray level suitable for the visual property of the human eye by use of logarithmic conversion and converts three primary colors (R, G, B) of light into three primary colors (Y, M, C) of toner. The UCR and BP portion 6 performs under-color removal from the converted data and black painting thereon and then enters the processed data into the memory 7. When copying is required, the digital image data is subsequently read out from the memory 7 (step d) and transferred to the laser unit 8 which outputs a full-color image.
As mentioned above, the conventional image forming device employs an advanced system that extracts a flesh-color area and a sky-blue area and separately corrects the extracted areas by using specially selected correction values in a correction table in order to match them with the visual properties of the human eye. This device, however, has a drawback that, if the device is used for color correction in a full-color digital copying system with a full-color digital scanner, it requires much time for processing and reduces the speed of color correction. An attempt to increase the processing speed of the device has led to its increased size with an increase in cost to manufacture because the color area extraction contains many operations such as removing separate points, expanding and contracting, labeling and so on. The problem is that any area of specific color, e.g., flesh-color must be extracted even if the original has only a small area of said color. In other words, the device always performs the extraction of the specified color areas on any kind of originals, e.g., an original mostly containing characters and having no need of the above-mentioned operations. In this case, the time is consumed with no effect on printing quality. The separate correction of an extracted area of any specified color, e.g., flesh-color, by using a factor selected for masking the color is effective to improve the color reproduction's quality but it makes the device expensive and time-consuming. This is the reason the device has not heretofore been employed in practical applications.
In a digital full-color copying machine, a full-color image copy to be output has several specified colors, e.g., human flesh-color and sky-blue, which must be faithfully reproduced. The specific colors are familiar to us in our daily life and, therefore, are easily recognized by the human eye. However, the conventional digital full color copier performs one-patterned color correction and offers poor reproduction of the specific colors. It cannot provide a full-color image with finely reproduced flesh-color and sky-blue color therein. This may totally effect the quality of the copy products of a the digital copying machine.